This invention relates to automatic power control of light sources. More particularly, this invention relates to gain normalization and automatic power control of modulated light sources.
Recently, there has been a dramatic increase in the number of technologies that are based on the transmission of lightwave signals. Some popular examples of these technologies include data retrieval systems such as DVDs, data transfer systems such as fiber optics, and data acquisition systems such as bar code scanners. Generally speaking, lightwave systems are desirable because they take advantage of the unique properties of light such as extended bandwidth, the ability to propagate long distances with little loss, and resistance to electro-magnetic interference (EMI).
Lightwave technology has revolutionized the transmission of electronic information. For example, fiber optic communications systems that use semiconductor lasers can attain data rates far in excess of that normally found in copper wire systems. Because the light intensity of a semiconductor laser is usually linearly proportional to an injected current, and the current in a photodetector is linearly proportional to incident optical intensity, data may transmitted as a modulation of the optical intensity. Such a lightwave system is analogous to a linear electrical link where current or voltage translates linearly into optical intensity. High speed semiconductor lasers and photodetectors enable intensity-modulation bandwidths greater than 10 GHz, allowing the development of a wide variety of radio and microwave frequency applications.
Converting microwaves into intensity-modulated light allows the use of optical fiber for transmission in place of bulky inflexible coaxial cable or microwave waveguide. Because signal attenuation in optical fiber is much less that of cable or waveguide, entirely new applications and architectures are possible. In addition, optical signals are usually tightly confined to the core of single-mode fiber, where it is immune to EMI, cross talk, or spectral regulatory control.
To achieve these advantages, several limitations must be overcome. The conversion of current to light intensity must be substantially linear. Several nonlinear mechanisms must be avoided by proper laser design or by the use of various linearization techniques.
An example of a conventional lightwave transmission system 10 with automatic power control is shown in FIG. 1. Transmission system 10 includes a lightwave emitter 110, a lightwave detector 120, a summing node 130, a reference signal 140, a fixed gain circuit 20, a modulator circuit 160, and an optical transmission medium 170.
In operation, a drive signal ID is applied to lightwave emitter 110 so that emitter 110 produces optical output signal 115. Optical output signal 115 is the principal output signal of circuit 10 which may be used to as optical data. As shown, principal signal 115 is applied to an optical transmission medium 170 for conveying the optical information. Another optical signal, monitor signal 116, is also generated. Signal 116 may be created directly by lightwave emitter 110 or by sampling a portion of principal output signal 115. Monitor signal 116 is applied to lightwave detector 120 to monitor the power of principal signal 115. This is accomplished by generating a feedback signal IFB that is representative of monitor signal 116. Principal signal 115 and monitor signal 116 are usually proportional to one another so the feedback signal IFB is proportionally related to principal signal 115.
As shown in FIG. 1, both feedback signal IFB and reference signal 140 (IREF) are coupled to summing node 130. Summing node 130 compares the feedback signal and the reference signal in order to produce a control signal IC that is proportional to the difference of these signals (sometimes referred to as an error signal). Fixed gain circuit 20, which is typically an integrator, receives control signal Ic and produces a bias signal IB in order to maintain emitter 110 at a power level that allows effective modulation. Bias signal IB is subsequently combined with modulation signal IM, which includes modulated information, to produce a drive signal ID that controls the output of lightwave emitter 110. In this way, the optical output of lightwave emitter 110 provides modulated optical communication signals to transmission medium 170.
When drive signal ID is initially applied to lightwave emitter 110, system 10 experiences wide variations in operating parameters. Three parameters of particular concern to circuit designers are the threshold level and slope efficiency of emitter 110 and the sensitivity of detector 120.
The threshold level of emitter 110 is generally defined as the magnitude of the drive signal ID required to produce a desired minimum light level. The slope efficiency of emitter 110 is generally defined as the derivative of optical output signal 115 with respect to the input signal (ID) when the input signal is above the threshold level. The sensitivity of detector 120 is generally defined as the ratio of the average value of the output signal (in this case the average value IFB) to the average optical output power (in this case, the average value of optical output signal 115).
The threshold level is indicative of the level above which lightwave emitter 110 generates a useful optical output signal. Maintaining emitter 110 above this threshold level is generally desirable in optical transmission systems because it reduces the response time of emitter 110 to drive signal ID, minimizes the amplitude of IM needed to produce the required optical modulation, and reduces duty cycle distortion.
The slope efficiency is representative of the amount of light produced by emitter 110 per unit drive signal ID. The value the slope efficiency directly affects the amplitude of the modulation signal IM needed to produce the necessary optical modulation.
Initially, it is necessary to compensate for variations in the threshold level and slope efficiency of light emitter 110 and for variations in sensitivity of detector 120. This is usually accomplished during the final assembly of transmission system 10 by selecting appropriate values for the amplitude of modulation signal IM and reference signal 140. Typically, reference signal 140 is adjusted until the average optical power produced by lightwave emitter 110 conforms to a given communications standard such as one promulgated by the IEEE (e.g., ETHERNET). Similarly, the amplitude of modulation signal IM is adjusted until the extinction ratio (i.e., the ratio between maximum instantaneous optical output power and minimum instantaneous optical output power) of optical output signal 115 is sufficiently high to conform with a given communication standard.
Over time and with changes in temperature, variations in the threshold level and slope efficiency cause the average power of optical output signal 115 to change. Summing node 130 senses this change through a corresponding change in feedback signal IFB and modifies control signal IC. This causes a change in bias signal IB to approximately correct for the change in optical output power. In this way, transmission system 10 maintains the average optical output power of lightwave emitter 110 approximately constant.
In addition to changes in average optical output power, a change in slope efficiency also adversely affects extinction ratio. This occurs because the change in slope efficiency changes the modulation amplitude of the optical output signal. This problem is commonly solved in system 10 by adjusting the amplitude of modulation signal IM to maintain the modulation amplitude of optical output signal 115 substantially constant.
One problem with system 10, however, is that variations in transfer ratio (i.e., the product of slope efficiency and detector sensitivity) reduce its ability to maintain the power of optical output signal 115 constant. In general, the accuracy with which any feedback control system, such as system 10, can maintain its output in proportion to its input is limited by its loop gain. High loop gain is required for high accuracy, but excessive loop gain causes stability problems. If the loop gain is too low, however, large errors will be encountered in controlling average optical power. Because the loop gain in system 10 is dependent on the transfer ratio, the accuracy and stability of system 10 varies as transfer ratio varies. Thus, as transfer ratio decreases, system accuracy is sacrificed, and when transfer ratio increases, stability is compromised.
System 10 experiences a decrease in transfer ratio due to initial manufacturing tolerances of emitter 110 and detector 120 and to drift in emitter 110. This causes several problems. Foremost among these is a decrease in system bandwidth which results in an undesirably long settling time when emitter 110 is turned on. If settling time is excessive, it takes too long to turn on and stabilize emitter 110. However, if bandwidth is too large, the servo loop (i.e., the circuitry that controls emitter 110) will attenuate low frequency content in the communications signal. Thus, such an overlap is undesirable.
To deal with these well known performance limitations, international standard agencies such as the IEEE have adopted communication protocols that allow for very long settling times and attempt to minimize the overlap problem by requiring the use of high overhead encoding schemes such as 8B10B. Such long settling times however, make burst transmissions, which save electrical power and improve eye safety, virtually impossible. Moreover, this solution prevents accurate data transfer during the start up period and significantly reduces overall data rates.
Thus, in view of the foregoing, it would desirable to provide a system and method for gain normalization and automatic power control for modulated light sources that overcomes the above-described and other deficiencies found in conventional systems.